Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $46.94   12.697%   $52.9   $52.90   $52.90   $52.90  
  Purchase #2 $7.75   6.725%   $8.2712   $8.27   $8.25   $8.30  
  Purchase #3 $237.91   13.366%   $269.7091   $269.71   $269.70   $269.70  
  Purchase #4 $69.74   11.227%   $77.5697   $77.57   $77.55   $77.60  
  Purchase #5 $71.69   8.347%   $77.674   $77.67   $77.65   $77.70  
  Purchase #6 $114.70   4.804%   $120.2102   $120.21   $120.20   $120.20  
  Purchase #7 $162.97   13.851%   $185.543   $185.54   $185.55   $185.50  
  7 purchase totals = $711.70       $791.8772   $791.87   $791.80   $791.90